As a filter for processing harmonic signals, there exists a filter to which a fundamental wave signal and harmonic signals having an integer multiple of a frequency of a fundamental wave signal are supplied and which suppresses the harmonic signals among these signals and outputs the fundamental wave signal.
The above-described filter is provided on an output portion of nonlinear elements such as an amplifier and a mixer. In a nonlinear element, together with an input of a fundamental wave signal, harmonic signals may be generated and output. In this case, there is the possibility that the harmonic signals cause electromagnetic interference to other components or systems. When the above-described filter is provided on an output portion of this nonlinear element, the harmonic signals can be suppressed.
As one of the above-described filters, there exists a filter including an input terminal, an output terminal, a transmission line connecting the input terminal and the output terminal, and an open-end stub configured to be provided corresponding to a supplied harmonic signal, coupled to the transmission line, and has a length corresponding to one quarter of a wavelength of the corresponding harmonic signal.
Each open-end stub makes short a connection node to the transmission line for a corresponding harmonic signal and suppresses the corresponding harmonic signal. On the other hand, each open-end stub makes open the connection node to the transmission line for the fundamental wave signal and passes the fundamental wave signal. As a result, the filter enables harmonic signals to be suppressed among the supplied fundamental wave signal and harmonic signals, and the fundamental wave signal to be produced from the output terminal.
FIG. 1 illustrates one example of this conventional filter. The illustrated conventional filter 100 has an input terminal 110, an output terminal 120, a transmission line 130 connecting the input terminal 110 and the output terminal 120, and open-end stubs 142, 143, 144, 145, and 146 coupled to the transmission line 130 through a connection node 131. Here, the characteristic impedance Z0 of the transmission line 130 is 50Ω.
To the input terminal 110 of the filter 100, a fundamental wave signal f0 and harmonic signals having an integer multiple of the frequency of the fundamental wave signal f0 are supplied. Here, suppose that a second harmonic signal 2f0 having twice the frequency, a third harmonic signal 3f0 having three times the frequency, a fourth harmonic signal 4f0 having four times the frequency, a fifth harmonic signal 5f0 having five times the frequency, and a sixth harmonic signal 6f0 having six times the frequency are supplied to the input terminal 110. Here, the frequency of the fundamental wave signal f0 is 1.300 GHz.
The open-end stubs 142 to 146 are provided respectively corresponding to the supplied harmonic signals 2f0 to 6f0, and have lengths corresponding to one quarter of wavelengths of the corresponding harmonic signals.
Here, the open-end stub 142 is provided corresponding to the second harmonic signal 2f0, and has a length (i.e., a length corresponding to one eighth of the wavelength λf0 of the fundamental wave signal f0) corresponding to one quarter of a wavelength λ2f0 of the second harmonic signal 2f0. The open-end stub 142 makes short the connection node 131 to the transmission line 130 for the second harmonic signal 2f0, and suppresses the second harmonic signal 2f0.
The open-end stub 143 is provided corresponding to the third harmonic signal 3f0, and has a length corresponding to one quarter of a wavelength λ3f0 of the third harmonic signal 3f0 (i.e., a length corresponding to one twelfth of the wavelength λf0 of the fundamental wave signal f0). The open-end stub 143 makes short the connection node 131 to the transmission line 130 for the third harmonic signal 3f0, and suppresses the third harmonic signal 3f0.
The open-end stub 144 is provided corresponding to the fourth harmonic signal 4f0, and has a length (i.e., a length corresponding to one sixteenth of the wavelength λf0 of the fundamental wave signal f0) corresponding to one quarter of a wavelength λ4f0 of the fourth harmonic signal 4f0. The open-end stub 144 makes short the connection node 131 to the transmission line 130 for the fourth harmonic signal 4f0, and suppresses the fourth harmonic signal 4f0.
The open-end stub 145 is provided corresponding to the fifth harmonic signal 5f0, and has a length (i.e., a length corresponding to one twentieth of the wavelength λf0 of the fundamental wave signal f0) corresponding to one quarter of a wavelength λ5f0 of the fifth harmonic signal 5f0. The open-end stub 145 makes short the connection node 131 to the transmission line 130 for the fifth harmonic signal 5f0, and suppresses the fifth harmonic signal 5f0.
The open-end stub 146 is provided corresponding to the sixth harmonic signal 6f0, and has a length (i.e., a length corresponding to one twenty-fourth of the wavelength λf0 of the fundamental wave signal f0) corresponding to one quarter of a wavelength λ6f0 of the sixth harmonic signal 6f0. The open-end stub 146 makes short the connection node 131 to the transmission line 130 for the sixth harmonic signal 6f0, and suppresses the sixth harmonic signal 6f0.
On the other hand, each open-end stub 142 to 146 makes open the connection node 131 to the transmission line 130 for the fundamental wave signal f0, and passes the fundamental wave signal f0. As a result, the filter 100 enables the harmonic signals 2f0 to 6f0 to be suppressed and the fundamental wave signal f0 to be produced from the output terminal 120 among the supplied fundamental wave signal f0 and harmonic signals 2f0 to 6f0 thereof.
As another filter using an open-end stub, there exists a filter which suppresses a fundamental wave signal using an open-end stub and passes a second harmonic signal (e.g., Japanese Laid-open Patent publication No. 2006-229840). Further, there exists a directional coupler (e.g., Japanese Laid-open Patent publication No. 2002-084113) using an open-end stub or a 3-multiple frequency circuit (e.g., Japanese Laid-open Patent publication No. 09-275319) using an open-end stub.
However, there is the possibility that the conventional filter using an open-end stub attenuates the fundamental wave signal f0 as a pass signal.